I previously asked Proof that $L=S=0$ for filled electron subshells? which motivated me to look more deeply into the restrictions the Pauli exclusion principle places on multi-particle angular momentum states.
It's well known that 2 distinguishable spin-1/2 particles could occupy up to 4 different states: $$ |++\rangle, |--\rangle, |+-\rangle, |-+\rangle $$
Or, in the angular momentum coupled basis:
\begin{align} |1, 1\rangle =& |++\rangle\\ |1, 0\rangle =& \frac{1}{\sqrt{2}}\left(|+-\rangle + |-+\rangle\right)\\ |1, -1\rangle =& |--\rangle\\ |0, 0\rangle =& \frac{1}{\sqrt{2}}\left(|+-\rangle - |-+\rangle\right) \end{align}
Here the top three states represent the total spin-1 triplet and the bottom state represents the spin-0 singlet. Notably, the triplet states a symmetric with respect to particle exchange while the singlet states are anti-symmetric with respect to particle exchange. Again, for distinguishable particles all 4 states are allowed. However, Fermions must respect the Pauli exclusion principle which says that the multi-particle state must be anti-symmetric with respect to particle exchange.
Thinking about this some more. Suppose we have, now, $N$ fermionic particles, each with intrinsic spin 1/2 and total orbital angular momentum $l=1$. This will be to build up to 6 electrons filling the $p$ shell. The single particle Hilbert space is then $\mathcal{H}_i$ and the total Hilbert space is
\begin{align} \mathcal{H} = \bigotimes_{i=1}^{N} \mathcal{H}_i \end{align}
The dimension of the single particle Hilbert space is $3\times 2 = 6$. The dimension of the multiparticle space is $6^N$. For $N$ from 1 to 6 this gives $\text{dim} = \left\{6, 36, 216, 1296, 7776, 46656\right\}$
However, for identical fermions the multiparticle Hilbert space is now the alternating tensor product of the single particle Hilbert space. The states are slater determinants of single particle states. My understanding is that the dimension of this space is given by
$$ _6 C_N = \frac{6!}{N!(6-N)!} $$
Since you must choose $N$ unique states from the set of 6 available single particle sates. This leads to the dramatically reduced dimensionalities of $\text{dim}=\left\{6, 15, 20, 15, 6, 1 \right\}$ for $N$ from 1 to 6.
I know immediately for $N=1$ that this Hilbert space decomposes to a total spin 1/2 and total spin 3/4 subspace since it is composed of one spin-1 and one spin-1/2 component. However, for 2 spins it is already complicated for me to determine the total spin subspaces. The only way I would know how to do it is first decompose the 36 dimensional subspace from the distinguishable case into total spin subspaces (I could do this without too much trouble) then explicitly write down the states of those subspaces and determine which ones are anti-symmetric and remove all the others. This would be terribly time consuming and wouldn't easily generalize to larger $N$. Alternatively I could write down all of the anti-symmetric states available, but then it's not obvious to me how to assign spin subspaces to particular states.
My questions are as follows. They are ordered from major to minor questions
- Is there a way to know, on symmetry or group theoretic ground, the angular momentum decomposition of the anti-symmetrized space based on the angular momentum of the constituent spaces or, even, the decomposition of the distinguishable Hilbert space?
- Similar to above, is there a group-theoretic way to determine whether a particular subspace of the total distinguishable Hilbert space is anti-symmetric, symmetric, or mixed symmetry?
- when you take the antisymmetric subspace of the distinguishable multi-particle Hilbert space are you guaranteed that your states always come in complete spin subspaces? Proof for this?
- Am I correct about the dimensionality of the alternating Hilbert space?
- How to notate the antisymmetric tensor product in Latex?
Even more succinctly: I know how to decompose a multi-particle Hilbert space into irreducible representations. What is a generic procedure to sort these irreducible representations based on their symmetrization properties?